Department of

# Mathematics

Seminar Calendar
for events the day of Tuesday, October 30, 2018.

.
events for the
events containing

Questions regarding events or the calendar should be directed to Tori Corkery.
    September 2018          October 2018          November 2018
Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa   Su Mo Tu We Th Fr Sa
1       1  2  3  4  5  6                1  2  3
2  3  4  5  6  7  8    7  8  9 10 11 12 13    4  5  6  7  8  9 10
9 10 11 12 13 14 15   14 15 16 17 18 19 20   11 12 13 14 15 16 17
16 17 18 19 20 21 22   21 22 23 24 25 26 27   18 19 20 21 22 23 24
23 24 25 26 27 28 29   28 29 30 31            25 26 27 28 29 30
30


Tuesday, October 30, 2018

11:00 am in 345 Altgeld Hall,Tuesday, October 30, 2018

#### The Gross-Hopkins duals of higher real K-theory at prime 2

###### Guchuan Li (Northwestern Math)

Abstract: The Hopkins-Mahowald higher real K-theory spectra $E_n^G$ are generalizations of real K-theory; they are ring spectra which give some insight into higher chromatic levels while also being computable. This will be a talk based on joint work with Drew Heard and XiaoLin Danny Shi, in which we compute that higher real K-theory spectra with group $G=C_2$ at prime $2$ and height $n$ are Gross-Hopkins self duals with a shift $4+n$. This will allow us to detect exotic invertible $K(n)$-local spectra.

12:30 pm in 441 Altgeld Hall,Tuesday, October 30, 2018

#### The coincidental down-degree expectations (CDE) phenomenon

###### Sam Hopkins   [email] (University of Minnesota)

Abstract: Let P be a finite poset. Consider two probability distributions on P: the uniform distribution; and the distribution where each element occurs with probability proportional to the number of maximal chains passing through it. Reiner, Tenner, and Yong observed that for many posets P of interest to algebraic combinatorialists, the expected Hasse diagram down-degree is the same for both these distributions. They called this the "coincidental down-degree expectations" (CDE) property for posets. I will review the history of the study of CDE posets: its origins in the algebraic geometry of curves, its connections to the tableaux/symmetric function/Schubert calculus world, etc. I will also present the "toggle perspective," which has been useful in establishing that various distributive lattices are CDE. Time permitting I will explain a forthcoming proof of a conjecture of Reiner-Tenner-Yong which says that certain weak order intervals corresponding to vexillary permutations are CDE. The proof involves expanding the "toggle perspective" to the setting of semidistributive lattices (following Barnard and Thomas-Williams).

1:00 pm in 345 Altgeld Hall,Tuesday, October 30, 2018

#### Surreal Substructures

###### Vincent Bagayoko (Paris 7)

Abstract: We study the class No of surreal numbers equipped with its natural order and simplicity relation. A "surreal substructure" of No is a subclass that is isomorphic to No itself for the restricted order and simplicity relation. Such surreal substructures frequently arise when studying various operations on the surreals such as algebraic operations, exponentiation, or infinite summation. This makes it worthwhile to investigate the general properties of surreal substructures in their own right. We will give their general properties, provide many examples, and focus on two applications of surreal substructures: to produce fixed points of certain specific surreal functions and to define classes of surreal numbers defined as simplest in a convex class of No. We will mention their use in studying the hyperserial structure of No conjectured by van der Hoeven.

2:00 pm in 243 Altgeld Hall,Tuesday, October 30, 2018

#### The sum-product problem

###### George Shakan (Illinois Math)

Abstract: The Erdős–Szemerédi sum-product problem asserts that for any $A$ in the integers either $|A+A|$ or $|AA|$ is at least $|A|^2$ up to an arbitrarily small power of $|A|$. In this talk, we'll discuss recent progress and further questions.

3:00 pm in 243 Altgeld Hall,Tuesday, October 30, 2018

#### Orientation data for coherent sheaves on local $\mathbb{P}^2$

###### Yun Shi (UIUC Math)

Abstract: Orientation data is an ingredient in the definition of Motivic Donaldson-Thomas (DT) invariant. Roughly speaking, it is a square root of the virtual canonical bundle on a moduli space. It has been shown that there is a canonical orientation data for the stack of quiver representations for a quiver with potential. In this talk, I will briefly introduce Motivic DT invariant, and the role of orientation data in its definition. I will then give a construction of orientation data for the stack of coherent sheaves on local $\mathbb{P}^2$ based on the canonical orientation data from quiver representations.